Conventional methods or processes for producing pellets or granules from a material or mixture of materials at least one of which is reactive basically involve steps of mixing the or each material with further ingredients such as binders, fillers and the like, optionally densifying the mix by light pressing and regrinding and final compaction or pressing of the reground material to the desired shape of pellet or granule. When these conventional methods are followed for materials such as calcium hydride for hydrogen production full fuel cells there can be problems with loss of a percentage of the hydrogen evolution capacity of the calcium hydride by the onset of an initial reaction between the calcium hydride in the pellet and any water vapour in the surrounding air. This in turn introduces inconsistency of length of reaction and amount of hydrogen subsequently evolved in a subsequent reaction such as in a fuel cell with ensuing variation in performance from pellet to pellet. The same problems can arise in a different manner if a pellet or granule comprising or incorporating the reactive material is used to evolve a gas in a liquid such as hydrogen evolution in water.